Average speed = Distance traveled / time taken
In this case Time taken = Difference in hours between 3 PM and 9 AM
= 6 hours
Total distance traveled = 350 km + 250 km
= 600 kilometers
So average speed = 600/6 = 100 km/hr
Average speed of bus = 27.78 m/s
So the bus's average speed = 27.78 m/s or 100 km/hr.